A cyclotron for accelerating ions (charged particles) in an outward spiral using an electric field impulse from a pair of electrodes and a magnet structure is disclosed in U.S. Pat. No. 1,948,384 (inventor: Ernest O. Lawrence, patent issued: 1934). Lawrence's accelerator design is now generally referred to as a “classical” cyclotron, wherein the electrodes provide a fixed acceleration frequency, and the magnetic field decreases with increasing radius, providing “weak focusing” for maintaining the vertical phase stability of the orbiting ions.
Modern cyclotrons are primarily of a class known as “isochronous” cyclotrons, wherein the acceleration frequency provided by the electrodes is likewise fixed, though the magnetic field increases with increasing radius to compensate for relativity; and an axial restoring force is applied during ion acceleration via an azimuthally varying magnetic field component derived from contoured iron pole pieces having a sector periodicity. Most isochronous cyclotrons use resistive magnet technology and operate at magnetic field levels from 1-3 Tesla. Some isochronous cyclotrons use superconducting magnet technology, in which superconducting coils magnetize warm iron poles that provide the guide and focusing fields required for acceleration. These superconducting isochronous cyclotrons operate at field levels from 3-5T. The present inventor worked on the first superconducting cyclotron project in the early 1980s at Michigan State University.
Cyclotrons of another class are known as synchrocyclotrons. Unlike classical cyclotrons or isochronous cyclotrons, the acceleration frequency in a synchrocyclotron decreases as the ion spirals outward. Also unlike isochronous cyclotrons, though like classical cyclotrons, the magnetic field in a synchrocyclotron decreases with increasing radius. The present inventor recently invented a high-field synchrocyclotron (described in U.S. Pat. Nos. 7,541,905 B2 and 7,696,847 B2) for proton beam radiotherapy and other clinical applications. Embodiments of this synchrocyclotron have warm iron poles and cold superconducting coils, like the existing superconducting isochronous cyclotrons, but maintain beam focusing during acceleration in a different manner that scales to higher fields and can accordingly operate with a field of, for example, about 9 Tesla.